Push ups are an awesome exercise. Very little space and no equipment required. If you have a few spare minutes, just drop down and do some. If you want a little variety, you can do them on you palms, your knuckles (fist) or fingertips. Try to go as deep as possible. See how many you can do in a row. If you get bored with that, you can try one-arm pushups. Splay the leg on your free arm side out to get some stability and then as you get stronger over time, try more and more to align your legs. If that gets boring, I know a guy who can do handstand pushups.
Uneven push-ups are a good option too. One hand on a small ball and one on the floor - then do proper push ups. It seems to me that these work different muscles than the handstand push ups, although I'm sure both are extremely beneficial to upper body strength.